Menu
AWS CloudFormation
User Guide (API Version 2010-05-15)

Conditions

The optional Conditions section includes statements that define when a resource is created or when a property is defined. For example, you can compare whether a value is equal to another value. Based on the result of that condition, you can conditionally create resources. If you have multiple conditions, separate them with commas.

You might use conditions when you want to reuse a template that can create resources in different contexts, such as a test environment versus a production environment. In your template, you can add an EnvironmentType input parameter, which accepts either prod or test as inputs. For the production environment, you might include Amazon EC2 instances with certain capabilities; however, for the test environment, you want to use reduced capabilities to save money. With conditions, you can define which resources are created and how they're configured for each environment type.

Conditions are evaluated based on input parameter values that you specify when you create or update a stack. Within each condition, you can reference another condition, a parameter value, or a mapping. After you define all your conditions, you can associate them with resources and resource properties in the Resources and Outputs sections of a template.

At stack creation or stack update, AWS CloudFormation evaluates all the conditions in your template before creating any resources. Any resources that are associated with a true condition are created. Any resources that are associated with a false condition are ignored.

Important

During a stack update, you cannot update conditions by themselves. You can update conditions only when you include changes that add, modify, or delete resources.

How to Use Conditions Overview

To conditionally create resources, you must include statements in at least three different sections of a template:

Parameters section

Define the input values that you want to evaluate in your conditions. Conditions will result in true or false based on values from these input parameter.

Conditions section

Define conditions by using the intrinsic condition functions. These conditions determine when AWS CloudFormation creates the associated resources.

Resources and Outputs sections

Associate conditions with the resources or outputs that you want to conditionally create. AWS CloudFormation creates entities that are associated with a true condition and ignores entities that are associated with a false condition. Use the Condition key and a condition's logical ID to associate it with a resource or output. To conditionally specify a property, use the Fn::If function. For more information, see Condition Functions.

Syntax

The Conditions section consists of the key name Conditions, followed by a single colon. Braces enclose all condition declarations. If you declare multiple conditions, they are delimited by commas.

Each condition declaration includes a logical ID and intrinsic functions that are evaluated when you create or update a stack. The following pseudo template outlines the Conditions section:

"Conditions" : {
  "Logical ID" : {Intrinsic function}
}

You can use the following intrinsic functions to define conditions:

  • Fn::And

  • Fn::Equals

  • Fn::If

  • Fn::Not

  • Fn::Or

Examples

The following sample template includes an EnvType input parameter, where you can specify prod to create a stack for production or test to create a stack for testing. For a production environment, AWS CloudFormation creates an Amazon EC2 instance and attaches a volume to the instance. For a test environment, AWS CloudFormation creates only the Amazon EC2 instance.

{
  "AWSTemplateFormatVersion" : "2010-09-09",

  "Mappings" : {
    "RegionMap" : {
      "us-east-1"      : { "AMI" : "ami-7f418316", "TestAz" : "us-east-1a" },
      "us-west-1"      : { "AMI" : "ami-951945d0", "TestAz" : "us-west-1a" },
      "us-west-2"      : { "AMI" : "ami-16fd7026", "TestAz" : "us-west-2a" },
      "eu-west-1"      : { "AMI" : "ami-24506250", "TestAz" : "eu-west-1a" },
      "sa-east-1"      : { "AMI" : "ami-3e3be423", "TestAz" : "sa-east-1a" },
      "ap-southeast-1" : { "AMI" : "ami-74dda626", "TestAz" : "ap-southeast-1a" },
      "ap-southeast-2" : { "AMI" : "ami-b3990e89", "TestAz" : "ap-southeast-2a" },
      "ap-northeast-1" : { "AMI" : "ami-dcfa4edd", "TestAz" : "ap-northeast-1a" }
    }
  },
    
  "Parameters" : {
    "EnvType" : {
      "Description" : "Environment type.",
      "Default" : "test",
      "Type" : "String",
      "AllowedValues" : ["prod", "test"],
      "ConstraintDescription" : "must specify prod or test."
    }
  },
  
  "Conditions" : {
    "CreateProdResources" : {"Fn::Equals" : [{"Ref" : "EnvType"}, "prod"]}
  },
  
  "Resources" : {
    "EC2Instance" : {
      "Type" : "AWS::EC2::Instance",
      "Properties" : {
        "ImageId" : { "Fn::FindInMap" : [ "RegionMap", { "Ref" : "AWS::Region" }, "AMI" ]}
      }
    },
    
    "MountPoint" : {
      "Type" : "AWS::EC2::VolumeAttachment",
      "Condition" : "CreateProdResources",
      "Properties" : {
        "InstanceId" : { "Ref" : "EC2Instance" },
        "VolumeId"  : { "Ref" : "NewVolume" },
        "Device" : "/dev/sdh"
      }
    },

    "NewVolume" : {
      "Type" : "AWS::EC2::Volume",
      "Condition" : "CreateProdResources",
      "Properties" : {
        "Size" : "100",
        "AvailabilityZone" : { "Fn::GetAtt" : [ "EC2Instance", "AvailabilityZone" ]}
      }
    }
  },
  
  "Outputs" : {
    "VolumeId" : {
      "Value" : { "Ref" : "NewVolume" }, 
      "Condition" : "CreateProdResources"
    }
  }  
}

The CreateProdResources condition evaluates to true if the EnvType parameter is equal to prod. In the sample template, the NewVolume and MountPoint resources are associated with the CreateProdResources condition. Therefore, the resources are created only if the EnvType parameter is equal to prod.

Related Resources

  • For more information about the syntax of each intrinsic function and how to associate them with resources, see Condition Functions.

  • For more information about input parameters, see Parameters.